1. Field of the Invention
The present invention relates to an optical transmission module and method which directly intensity-modulate a semiconductor laser to carry out optical transmission, and more particularly to an optical transmission module connected to a transmission path, which includes a reflection point having a predetermined reflection coefficient, and an optical transmission method for setting a reflectivity condition of the optical module for carrying out optical transmission.
2. Description of the Background Art
Conventionally, through the rapid spread of the Internet, the transmission capacity of data communication networks has been increased. While the transmission capacity has been increased, middle- and long-distance systems, typified by metro networks, have become bottlenecks of the communication networks, increasing the need for techniques and optical devices which economically realize faster middle- and long-distance systems. As for short-distance data transmission, through standardization activities of 10-Gigabit-Ethernet, etc., reduction in size and cost of optical devices is actively sought.
As a light emitting device, which is a key device for determining an optical transmission efficiency, a distributed feedback semiconductor laser is widely used. Semiconductor lasers, which are light emitting devices for use in middle- and long-distance transmission through single mode fibers, achieve a satisfactory transmission quality and broadband characteristics because of their narrow spectrum characteristics. In the case of using an ordinary semiconductor laser together with a transmission path which includes a reflection point, it is necessary to provide the semiconductor laser with an optical isolator component for the purpose of achieving a satisfactory system efficiency, while reducing interference noise. For example, the optical isolator component is composed of a combination of a Faraday element having a magneto-optic effect and a polarizer. Even if the semiconductor laser is not provided with the optical isolator component, it is possible to reduce interference noise in a multiplexed electrical signal. However, in the case of ordinary middle- or long-distance high-speed baseband optical transmission, where a narrow spectrum width is required, an interference noise level is increased, and therefore the usefulness of such a semiconductor laser is low.
Hereinbelow, a conventional optical transmission/reception system including an optical isolator component is described. FIG. 6 is a block diagram showing a schematic structure of the optical transmission/reception system, and FIG. 7 is a diagram used for explaining how an optical signal propagates through the optical transmission/reception system shown in FIG. 6.
In FIG. 6, the conventional optical transmission/reception system includes a drive unit 101, a semiconductor laser 102, an aspherical lens 103, an optical isolator 104, a transmission path 105, and an optical receiver 106. If an input signal Si is inputted to the drive unit 101, the drive unit 101 converts the inputted signal into a modulation signal Sm having a predetermined extinction ratio to directly intensity-modulate the semiconductor laser 102. The semiconductor laser 102 performs electrical/optical conversion on the modulation signal Sm to generate signal light Ls1. The signal light Ls1 is outputted from the semiconductor laser 102 and collected by, for example, the aspherical lens 103 to be coupled through the optical isolator 104 to an input end of the transmission path 105. After propagating through the transmission path 105, the signal light Ls1 is outputted as signal light Ls2 from an output end of the transmission path 105, and received by the optical receiver 106.
Here, a reflection point X having a predetermined reflection coefficient is formed on the transmission path 105 due to influences of connection portions with optical connectors, etc. As shown in FIG. 7, the signal light Ls1 having been inputted to the transmission path 105 propagates on the transmission path 105, and is partially reflected at the reflection point X. A portion of the signal light Ls1 that has not been reflected at the reflection point X is inputted as direct light Ld into the optical receiver 106. On the other hand, reflection light Lr, which has been reflected at the reflection point X on the transmission path 105 to propagate in a direction opposite to that of the signal light Ls1, is re-reflected by an output end surface, a laser active region, etc., of the semiconductor laser 102. Thereafter, the re-reflected reflection light (multiplexed reflection light) Lr is outputted as the signal light Ls2 together with the direct light Ld from the output end of the transmission path 105 to the optical receiver 106.
However, the reflection light Lr, which has been reflected at the reflection point X on the transmission path 105 to propagate in the direction opposite to that of the signal light Ls1, is weakened by the optical isolator 104, making it possible to prevent the reflection light Lr from being re-reflected by the output end surface, the laser active region, etc., of the semiconductor laser 102. That is, it is possible to prevent the multiplexed reflection light Lr from occurring and being outputted to the optical receiver 106. Accordingly, it is possible to prevent interference noise from occurring in the optical receiver 106 due to detection of the direct light Ld and the multiplexed reflection light Lr from the signal light Ls2.
Japanese Laid-Open Patent Publication No. 2000-47069 discloses a semiconductor laser module in which an output end surface of a semiconductor laser makes an angle with optical fibers. In the semiconductor module, the angle between the output end surface of the semiconductor laser and the optical fibers prevents the reflection light from being re-reflected by the output end surface, a laser active region, etc., of the semiconductor laser.
Regarding the optical transmission/reception system including the optical isolator 104, the cost of the optical isolator 104 is high, and therefore it is difficult to reduce the cost of the optical transmission module including the optical isolator 104.
As for the semiconductor laser module disclosed in Japanese Laid-Open Patent Publication No. 2000-47069, high accuracy is required for adjusting optical coupling conditions with the angled optical fibers. That is, there is a difficulty in optically coupling the semiconductor laser with the optical fibers, resulting in an increase in cost of the optical transmission module.